Effect of stocking density and group size on growth performance, carcass traits and meat quality of outdoor-reared rabbits

Meat Science 93 (2013) 162–166

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Effect of stocking density and group size on growth performance, carcass traits and meat quality of outdoor-reared rabbits Gisella Paci a,⁎, Giovanna Preziuso a, Maria D’Agata a, Claudia Russo a, Antonella Dalle Zotte b a b

Department of Veterinary Science, University of Pisa, Viale delle Piagge 2, 56124 Pisa, Italy Department of Animal Medicine, Production and Health, University of Padova, Agripolis, Viale dell’Università 16, 35020 Legnaro, Padova, Italy

a r t i c l e

i n f o

Article history: Received 20 October 2011 Received in revised form 6 August 2012 Accepted 13 August 2012 Keywords: Rabbit Outdoor rearing system Stocking density Group size Growth Meat quality

a b s t r a c t The effect of stocking density (16 rabbits/m 2, 5 rabbits/m 2, 2.5 rabbits/m 2, n = 60, Experiment 1) and group size (4 rabbits/cage, 8 rabbits/cage, 16 rabbits/cage, n = 88, Experiment 2) on productive performance, carcass and meat quality of a slow-growing rabbit population reared outdoors was investigated in two experiments. The highest stocking density induced the highest skin percentage. Lower stocking densities showed lower lightness of Biceps femoris and higher redness of Longissimus lumborum muscles. Four rabbits/cage group (Experiment 2) showed the highest daily weight gain and slaughter weight and the lowest skin percentage. The muscles of 16 rabbits/cage showed significantly higher pHu than 8 and 4 rabbits/cage. BF of 16 and 4 rabbits/cage showed higher L* value. Productive performance and meat quality of rabbits reared outdoors improved in low group size while stocking density needs more experiments. The best combination of density, group size and total available surface that showed the best production and carcass traits was of 5 rabbits/m 2, 4 rabbits/cage, and 0.8 m2. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Interest in obtaining rabbit meat from less intensive rearing systems has increased in the last decade. Several studies have been carried out to evaluate alternative methods for the management of intensive rabbit production, analyzing the effect of cage size, floor types, group size, and stocking densities on productive performance, behaviour, and carcass and meat quality of commercial hybrids (Dal Bosco, Castellini, & Mugnai, 2002; Princz et al., 2008; Szendrő & Dalle Zotte, 2011; Szendrő et al., 2009; Trocino & Xiccato, 2006). When outdoor housing systems are adopted, the environmental conditions may present high variability and therefore local breeds or populations highly adaptable to both environmental variability and unfavourable environmental conditions are better suited than commercial hybrids (D'Agata et al., 2009; Lambertini, Vignola, & Zaghini, 2001; McNitt, Way, Way, & Forrester-Anderson, 2003; Szendrő & Dalle Zotte, 2011). Some researches have been conducted to study the effect of outdoor alternative rearing system on the productive performance of autochthonous populations, but not enough knowledge has yet been acquired to provide clear information on the right group size or stocking density of rabbits reared outdoors (Cavani et al., 2004; D'Agata et al., 2009). The aim of this study was to investigate the effect of different stocking density and group size on productive performance, carcass,

⁎ Corresponding author. Tel.: +39 050 2216903; fax: +39 050 2216901. E-mail address: [email protected] (G. Paci). 0309-1740/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.meatsci.2012.08.012

and meat quality of a local rabbit population characterized by slow growth reared outdoors. 2. Materials and methods The effects of stocking density and group size were studied separately in two experiments and are discussed separately. Grey-coloured local rabbits (agouti or wild-type) were utilized: this rabbit population has been reared for more than thirty years in several small sized farms in central Italy (Tuscany). At the Rabbitry Station of Pisa University, about 20 females and 10 males, representative of this local population, were bred for ten years. This population had the following characteristics: high rusticity, fertility about 90%, total born/delivery 8.2 ±2.8, adult live weight 4000±100 g (male) and 3400±200 g (female), weaning weight 890±155 g (35d), slaughter weight 2500±300 g (101±2d), and dressing out percentage about 60% (D'Agata et al., 2009). 2.1. Animals and housing system 2.1.1. Experiment 1: stocking density Sixty 51-day-old male rabbits were divided into three experimental groups of 20 rabbits of similar average body weights and reared in wire net floor colony cages at the same group size (4 animals/cage) but at different stocking density as follows: 16 rabbits/m2 (cage size: cm 50 × 50 × 76 h), 5 rabbits/m 2 (cage size: cm 100 × 80× 76 h) and 2.5 rabbits/m 2 (cage size: cm 100 × 160× 76 h). Cages were located in an outdoor pen built to provide the animals with protection against predators and the radiation of the sun.

G. Paci et al. / Meat Science 93 (2013) 162–166

Colony cages were equipped with feeders and nipple drinkers. The animals received the same dietary treatment throughout the trial (pelleted feed and alfalfa hay ad libitum) until 103 days of age. From 51 to 103 days of age, individual body weights and pellet intake per cage were recorded weekly, and daily weight gain and feed to gain ratios were calculated. 2.1.2. Experiment 2: group size Eighty-eight 56-day-old male rabbits were divided into three experimental groups of similar average body weights and reared in wire net floor colony cages in an outdoor pen at the same stocking density (5 rabbits/m 2) but at different group sizes; in order to maintain the same density, group sizes were obtained by using different cage sizes as follows: 4 rabbits/cage (cage size: cm 100 × 80 × 76 h; 4 replicates: n = 16); 8 rabbits/cage (cage size: cm 100 × 160× 76 h; 3 replicates: n =24); 16 rabbits/cage (cage size: cm 100 × 320 × 76 h; 3 replicates: n =48). Colony cages were equipped with feeders and nipple drinkers. Throughout the trial and up to 103 days of age, the animals received the same diet of pelleted feed and alfalfa hay ad libitum used in Experiment 1; individual body weights and pellet intake per cage were recorded weekly, and daily weight gain and feed to gain ratios were calculated. 2.2. Slaughter traits and muscle sampling At 103 days of age, 12 rabbits per group were weighed (SW), electrically stunned and slaughtered at an EU-licensed abattoir. The slaughtering and carcass dissection procedures followed the World Rabbit Science Association (WRSA) recommendations described by Blasco and Ouhayoun (1993). The slaughtered rabbits were bled, and the skin, genitals, urinary bladder, gastrointestinal tract and the distal part of the legs were removed. Carcasses (with head, thoracic cage organs, liver, kidneys, perirenal and scapular fat) were weighed, then chilled at +4 °C for 24 h in a ventilated room. The chilled carcasses were weighed (CCW), and the head, thymus, trachea, esophagus, heart, lungs, liver and kidneys were removed to obtain the reference carcasses (RCW). The dressing out percentage (CCW as percentage of SW) and the ratio of head and liver and carcass parts to either CCW or RCW were calculated as required. The RCW was divided into joints at the left and right hind legs and loin region (between the 1st and the 7th lumbar vertebra). The left hind legs were deboned and the meat-to-bone ratio was calculated according to Blasco and Ouhayoun (1993). The right hind legs and both sides of the Longissimus lumborum (LL) muscle were used to determine the meat quality parameters.

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for proximate composition (AOAC, 1995). For this purpose, the samples were freeze-dried. Chemical data were processed by WINISI software to improve a previous calibration set and to predict the proximate composition of the non-chemically analyzed samples (Berzaghi, Dalle Zotte, Jansson, & Andrighetto, 2005; Dalle Zotte, Berzaghi, Jansson, & Andrighetto, 2006). Protein content, including glucidic molecules and their catabolites (0.25%), was calculated by difference in accordance with AOAC (1995). TBARS (thiobarbituric acid reactive substances) were measured on the meat from the left hind leg using the following method: 5 g of minced muscle was homogenized with 15 ml of a TCA solution (trichloroacetic acid) and DTPA (diethylenetriaminepentacetic acid) in distilled water for 2 min. After distillation, 4 ml of distilled was added to 4 ml TBA reagent, heated in boiling water for 60 min, and then cooled under running tap water for 10 min. Absorbance was measured at 532 nm against a blank. Lipid oxidation products were quantified as malondialdehyde (MDA) equivalents (mg/kg of muscle) (Ke, Ackman, Linke, & Nash, 1977; Tarladgis, Watts, & Younathan, 1960). 2.4. Statistical analysis Mean and standard error were calculated for all quantified variables. Live performances and meat quality parameters were evaluated by PROC ANOVA using the following model: Yij ¼ μ þ α i þ εij For feed intake and feed to gain ratio variables, each cage was an experimental unit. Slaughtering traits were analysed as weight by a GLM procedure of the SAS statistical package (SAS, 2002) and expressed as percentage. The model was: Yijk ¼ μ þ ai þ bxij þ εijk where: μ α x ε

general mean; stocking density or group size effect (i = 1–2–3); covariate: reference variable; residual error.

The statistical significance of the differences was assessed with the Tukey test (SAS, 2002).

2.3. Meat quality parameters 3. Results and discussion The ultimate pH (pHu) was determined in situ on the LL muscle at the level of the 5th lumbar vertebra and on the Biceps femoris (BF) muscle with a portable pH-meter (Hanna) equipped with a glass electrode (3 mm diameter conic tip) suitable for meat penetration. At 24 h post mortem, instrumental meat colour expressed as L* (lightness), a* (redness), b* (yellowness) according to the CIElab system (CIE, Commission Internationale d'Eclairage, 1976) was measured with a Minolta CR300 chromameter (Minolta, Osaka, Japan) on a transversal section of the LL muscle and on the BF muscle surface. The illuminant was D65 and an incidence angle of 0 was used. The values corresponded to the average of three measurements per sample. Water-holding capacity was measured as cooking loss after cooking in a ventilated oven (163 °C to a core temperature of 71 °C) and in a water bath (80 °C for 2 h) on samples of LL muscle (AMSA, 1995; Honikel, 1998). The raw meat from the left hind leg was ground and scanned in duplicate with Near Infrared Reflectance Spectroscopy (NIRS) using a Foss NIRSystems 5000 system. Ten samples were selected by NIR

3.1. Effect of stocking density (Experiment 1) 3.1.1. Growth performance During the trial, only one rabbit in the 16 rabbits/m2 group and one in the 2.5 rabbits/m 2 group died from diarrhea (Table 1). This mortality value is considered low for rearing rabbits because mortality generally reaches 10% in intensive indoor rearing conditions. Stocking density did not affect mortality, in agreement with previous studies (Aubret & Duperray, 1992; Maertens & De Groote, 1984; Szendrő & Luzi, 2006). Rabbits growth performance did not show any significant difference among the experimental groups: this result may depend on the limited number of rabbits used for the experiment and on the high variability observed, the latter derived from the use of unselected animals. However, recent literature reports that when stocking density is lower than 15–17 rabbits/m 2, only random effects are observed on rabbit growth performance (Szendrő & Dalle Zotte, 2011;

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Table 1 Effect of stocking density on growth performance (Experiment 1).

Initial rabbits, no. Body weight (51 d), g Dead rabbits (51–103 d), no. Body weight (103 d), g Daily weight gain, g/d Cages, no. Complete feed intake, g/d Feed to gain ratio

16 rabbits/m2

5 rabbits/m2

2.5 rabbits/m2

20 1015 ± 29.3 1 2502 ± 38.3 28.7 ± 0.81 5 145.0 ± 1.42 5.05 ± 0.378

20 1025 ± 25.7 0 2567 ± 41.3 29.7 ± 0.72 5 147.0 ± 0.99 4.95 ± 0.615

20 970 ± 27.1 1 2479 ± 50.5 29.0 ± 0.77 5 147.1 ± 2.31 5.07 ± 0.106

±: Standard error of the least squares means.

Szendrő et al., 2009). Other studies reported that it is not the number of animals per m 2 but rather the total weight of the animals per m 2 that induces lower productive performance when density reaches or exceeds 45 kg/m 2 (Aubret & Duperray, 1992; Maertens & De Groote, 1984; Szendrő & Dalle Zotte, 2011); in this study the total weight per m 2 remained below this limit, reaching 40 kg/m 2 only for the 16 rabbits/m 2 group, without any negative effect on productive traits. In all groups, the feed to gain ratio was relatively high: this could be related to the local population of rabbits utilized in the study, which is characterized by slow growth; however it is necessary to take into account that only pellet intake was measured but rabbits also consumed alfalfa hay, which reduced the feed's digestibility (Ouhayoun, 1998).

3.1.2. Carcass and meat quality With regard to carcass traits (Table 2), only the skin percentage was significantly affected by the stocking density: rabbits reared with the highest stocking density (16 rabbits/m2) had the highest skin percentage (P b 0.01). This result could be related to the limited animal movements at high stocking density combined with a low floor surface. It is probable that together with the increase of perirenal fat content (Table 2), also subcutaneous fat increased, leading to an increase in skin incidence. Recently, Gondret, Hernandez, Rémignon, and Combes (2009) proved that subjecting growing rabbits to jumping exercises for 5 weeks significantly increased the proportions of the hind parts of their bodies in comparison to rabbits that were not exercised. On the other hand, Dal Bosco, Castellini, and Bernardini (2000) and Pla (2008) reported that as stocking density increases, hind leg proportion decreases on hybrid rabbits. A similar tendency was observed in our study, but the contribution of stocking density and space availability has yet to be quantified.

Table 2 Effect of stocking density on carcass traits (Experiment 1). 16 rabbits/m2 5 rabbits/m2 2.5 rabbits/m2 Rabbits, no. 12 Slaughter weight (SW), g 2485 ± 34.8 Skin, % SW 16.4 ± 0.21A Full gastrointestinal tract, % SW 17.1 ± 0.25 Chilled carcass weight (CCW), g 1478 ± 20.2 Dressing out, % 59.5 ± 0.39 Head, % CCW 9.6 ± 0.24 Liver, % CCW 5.3 ± 0.18 Reference carcass weight (RCW), g 1218 ± 19.1 Perirenal fat, % RCW 1.78 ± 0.281 Loin, % RCW 20.8 ± 0.35 Hind leg, % RCW 33.4 ± 0.27 Meat-to-bone ratio 4.0 ± 0.27

12 2560 ± 43.6 15.4 ± 0.18B 17.4 ± 0.40 1510 ± 23.4 59.0 ± 0.57 9.4 ± 0.23 5.3 ± 0.17 1242 ± 20.4 1.34 ± 0.207 21.3 ± 0.34 34.2 ± 0.14 3.6 ± 0.28

12 2495 ± 48.1 15.2 ± 0.22B 16.4 ± 0.36 1512 ± 22.6 60.7 ± 0.51 9.4 ± 0.13 5.2 ± 0.14 1249 ± 19.3 1.33 ± 0.193 21.0 ± 0.39 34.1 ± 0.36 4.0 ± 0.36

±: Standard error of the least squares means. A, B : means in the same row with no common superscripts differ significantly (P b 0.01).

Hind leg meat-to-bone ratio was favourable for all experimental groups and similar to other studies conducted on the same rabbit population (D'Agata et al., 2009). Meat quality parameters are shown in Table 3. LL and BF muscles of 5 or 2.5 rabbits/m 2 groups showed lower lightness (L* value) than the 16 rabbits/m 2 group, reaching a statistically significant level in BF muscle (P b 0.01). The redness (a* value) of LL muscle was significantly higher in the 5 and 2.5 rabbits/m2 groups than in the 16 rabbits/m 2 group (P b 0.01). The lower L* value and the higher a* value observed in meats derived from rabbits reared at a stocking density lower than 16 rabbits/m2 may depend on an increase in muscle oxidative metabolism, which was mainly attributable to the presumed increase in animal locomotory activity as formerly reported by other authors (Gondret et al., 2009; Gregory, 2003; Pla, 2008). Considering that growth performance was not affected by the stocking densities applied, as expected, no significant differences were found in meat proximate composition or TBARS values. Previous commercial hybrid rabbit studies (Dalle Zotte, 2002; Szendrő & Dalle Zotte, 2011) have reported that meat proximate composition does not vary when stocking density is maintained lower than 17 rabbits/m2. 3.2. Effect of group size (Experiment 2) 3.2.1. Growth performance Throughout this trial, no mortality was observed, but the occurrence of aggressiveness induced us to remove one rabbit from the 8 rabbits/cage group and three rabbits from the 16 rabbits/cage group during the last fifteen days of the trial (Table 4). Some researchers have shown that in larger groups, the aggressiveness of animals increases with increasing age due to the onset of sexual development. The percentage of aggressive animals could be the same in small and large groups, but in larger groups, an aggressive rabbit can injure more cage-mate animals (Princz et al., 2008; Szendrő et al., 2009). A recent study has also proved that housing growing rabbits in pens of 6 animals each (16 rabbits/m 2) segregated by sex, primarily only males, is disadvantageous for the increased occurrence of aggressive behaviour from 10 weeks of age onward, even though sex segregation had no effect on animal growth performance (Szendrő, Gerencsér, Odermatt, Dalle Zotte, Zendri, Radnai, Nagy, & Matics, 2012). In our study, the animals' age was 3 weeks higher Table 3 Effect of stocking density on quality parameters of Longissimus lumborum (LL) and Biceps femoris (BF) muscles and on hind leg meat (Experiment 1). 16 rabbits/m2 Meat samples, no. 12 LL muscle pHu 5.7 ± 0.05 Colour L* 58.3 ± 0.87 a* 1.6 ± 0.22B b* 1.7 ± 0.23 Oven cooking loss, % 13.4 ± 0.69 Water bath cooking loss, % 17.8 ± 0.82 BF muscle pHu 6.0 ± 0.08 Colour L* 55.0 ± 0.73A a* 3.3 ± 0.22 b* 3.4 ± 0.99 Hind leg proximate composition Water, % 74.33 ± 0.094 Protein, % 22.94 ± 0.060 Lipids, % 1.51 ± 0.104 Ash, % 1.27 ± 0.017 TBARS, mg MDA/kg 0.23 ± 0.022

5 rabbits/m2

2.5 rabbits/m2

12

12

5.7 ± 0.06

5.7 ± 0.04

56.4 ± 0.80 2.4 ± 0.22A 1.5 ± 0.18 15.0 ± 0.96 16.3 ± 0.79

57.2 ± 0.85 2.9 ± 0.39A 2.1 ± 0.26 15.1 ± 0.87 17.6 ± 0.67

6.0 ± 0.05 51.4 ± 0.66 4.0 ± 0.38 3.1 ± 0.98

5.9 ± 0.05 B

74.35 ± 0.168 23.03 ± 0.134 1.38 ± 0.100 1.29 ± 0.022 0.22 ± 0.020

52.3 ± 0.55B 3.9 ± 0.36 3.2 ± 0.93 74.59 ± 0.193 22.93 ± 0.197 1.25 ± 0.097 1.23 ± 0.012 0.22 ± 0.019

±: Standard error of the least squares means. A, B : Means in the same row with no common superscripts differ significantly (P b 0.01).

G. Paci et al. / Meat Science 93 (2013) 162–166 Table 4 Effect of group size on growth performance (Experiment 2).

Initial rabbits, no. Body weight (56 d), g Removed rabbits, no Body weight (103 d), g Daily weight gain, g/d Cages, no. Complete feed intake g/d Feed to gain ratio

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Table 5 Effect of group size on carcass traits (Experiment 2).

4 rabbits/cage

8 rabbits/cage

16 rabbits/cage

16 1383 ± 49.3 0 2795 ± 55.8A 30.1 ± 1.03A 4 137.7 ± 6.81 4.6 ± 0.30

24 1267 ± 40.2 1 2469 ± 46.4B 25.5 ± 0.84B 3 119.5 ± 7.86 4.9 ± 0.34

48 1344 ± 28.5 3 2533 ± 33.2B 25.3 ± 0.61B 3 149.1 ± 7.87 5.9 ± 0.34

±: Standard error of the least squares means. A, B : Means in the same row with no common superscripts differ significantly (P b 0.01).

than that currently used for commercial rabbits and, even though the stocking density was lower (5 rabbits/m 2) and floor area was higher (>1.6 m 2) than that used by Szendrő et al. (2012) (0.45 m 2), the aggressiveness exhibited by the animals of the 2 largest groups seems to indicate a predominance of group size effect. Body weight at slaughter (103 days of age) was affected by group size (P b 0.01) and probably due to the different disposable floor space because in order to maintain the same density, group sizes were reached using various cage sizes. Rabbits housed 4 rabbits/cage showed higher body weights at slaughter than those housed 8 or 16 rabbits/cage: these differences in body weights were 13% and 10% higher than those of rabbits housed 8 and 16 rabbits/cage, respectively. The 4 rabbits/cage group also showed a significantly higher daily weight gain than that of the 8 and 16 rabbits/cage groups (P b 0.01). The differences in growth performance observed among the 3 groups are probably attributable to both factors, group size and available space, which affected aggressive behaviours and locomotory activity, respectively. In detail, the higher weight gain and body weight exhibited by rabbits housed at lower group size (4 rabbits/m 2) but also characterized by a lower floor area availability (0.8 m 2) are probably the result of their concurrent lower aggressiveness (due to the low group size) and locomotory activity (due to the low space availability). The decrease in these parameters when group size increases has been widely demonstrated in commercial rabbits (Combes, Postollec, Cauquil, & Gidenne, 2010; Dal Bosco et al., 2002; Dalle Zotte et al., 2009; Lambertini et al., 2001; Szendrő & Dalle Zotte, 2011; Szendrő et al., 2009). Regarding feed to gain ratio, all groups showed high values, similar to those observed in a previous study of the same population (D'Agata et al., 2009): a high feed to gain ratio is typical of unselected populations characterized by slow growth rate, but a high feed to gain ratio could be also related to both the less controlled environmental conditions of the outdoor rearing system and to the fiber-rich diet fed, partly as hay. The feed to gain ratio observed in the 16 rabbits/cage group tended to be higher than in the 8 and 4 rabbits/cage groups. Overall, a number of studies has found that when group size exceeds seven rabbits per cage, the feed conversion ratio worsens (Princz et al., 2009; Szendrő & Dalle Zotte, 2011) . 3.2.2. Carcass traits and meat quality Among the slaughter variables considered (Table 5), only slaughter weight, skin percentage and hind leg meat-to-bone ratio showed significant differences. The 4 rabbits/cage group had the highest slaughter weight and a lower skin percentage than the 8 and 16 rabbits/cage groups (Pb 0.01). Hind leg meat-to-bone ratio was higher in the 4 and 16 rabbits/cage groups than in the 8 rabbits/cage group (Pb 0.05). In the 4 rabbits/cage group, the higher hind leg meat-to-bone ratio probably depended on the rabbits' faster growth (Ouhayoun, 1998). The similar high meat-to-bone ratio found in the group with 16 rabbits/cage could be explained by the rabbits' more intensive physical exercise afforded by

Rabbit, no. Slaughter weight (SW), g Skin, % SW Full gastrointestinal tract, % SW Chilled carcass weight (CCW), g Dressing out, % Head, % CCW Liver, % CCW Reference carcass weight (RCW), g Perirenal fat, % RCW Loin, % RCW Hind leg, % RCW Meat-to-bone ratio

4 rabbits/ cage

8 rabbits/ cage

16 rabbits/ cage

12 2747 ± 36.3A 16.4 ± 0.21B 16.1 ± 0.47 1678 ± 25.6 61.0 ± 0.45 8.1 ± 0.19 5.0 ± 0.13 1415 ± 22.9 1.35 ± 0.166 21.8 ± 0.43 34.7 ± 0.22 4.8 ± 0.16a

12 2444 ± 35.4B 17.6 ± 0.14A 16.2 ± 0.32 1459 ± 24.2 60.1 ± 0.29 7.9 ± 0.19 4.9 ± 0.23 1239 ± 18.7 1.38 ± 0.294 22.4 ± 0.54 34.3 ± 0.28 3.8 ± 0.16b

12 2568 ± 30.6B 18.0 ± 0.29A 15.3 ± 0.48 1559 ± 22.9 61.0 ± 0.34 8.3 ± 0.18 5.1 ± 0.21 1314 ± 20.1 0.97 ± 0.198 21.6 ± 0.18 34.8 ± 0.18 4.6 ± 0.25a

±: Standard error of the least squares means. A, B : means in the same row with no common superscripts differ significantly (P b 0.01). a, b : means in the same row with no common superscripts differ significantly (P b 0.05).

the larger cage size utilized to ensure the same stocking density (Dal Bosco et al., 2002; Gondret et al., 2009; Pla, 2008). Meat quality traits are reported in Table 6. The LL and BF muscles of the 16 rabbits/cage group showed significantly higher pHu values than the 8 and 4 rabbits/cage groups (P b 0.01 and P b 0.05, for LL and BF muscles, respectively). However, several studies have found that rabbit meat pHu tended to decrease with increasing group size (see review by Szendrő & Dalle Zotte, 2011). These authors have suggested that the stressful living conditions created by the aggression of certain rabbits in each group and general social, marking and agonistic behaviour could induce an adaptive stress response in the muscle in order to control the greater amount of free radicals produced by metabolism. For this reason, rabbits housed in large group size could have provided better response to pre-slaughter treatments and reduced the consumption of the glycogen available for subsequent post-mortem glycolysis, in this way lowering pHu. In our study, on the other hand, it is likely that the larger cage size utilized in the 16 rabbits/cage group to maintain the same stocking density as the other 2 groups induced greater locomotory activity in the rearing period and during animal capture before slaughtering: these situations could have caused a decrease in muscular glycogen Table 6 Effect of group size on quality parameters of Longissimus lumborum (LL) and Biceps femoris (BF) muscles and on hind leg meat (Experiment 2). 4 rabbits/cage Meat samples, no. 12 LL muscle pHu 5.6 ± 0.02B Colour L* 54.9 ± 0.46 a* 1.2 ± 0.15 b* 1.5 ± 0.18 Oven cooking loss, % 15.6 ± 0.85 Water bath cooking loss, % 17.8 ± 0.83 BF muscle pHu 5.7 ± 0.04b Colour L* 54.1 ± 0.66A a* 2.8 ± 0.22 b* 3.1 ± 0.24 Hind leg proximate composition Water, % 74.95 ± 0.161B Protein, % 22.52 ± 0.117 Lipids, % 1.33 ± 0.067 Ash, % 1.28 ± 0.14 TBARS, mg MDA/kg 0.29 ± 0.025

8 rabbits/cage

16 rabbits/cage

12

12

5.6 ± 0.06B

5.8 ± 0.05A

55.5 ± 0.97 1.7 ± 0.27 2.0 ± 0.27 15.6 ± 0.53 17.5 ± 1.48

56.5 ± 0.59 1.4 ± 0.25 1.8 ± 0.13 14.7 ± 0.57 17.2 ± 0.75

5.7 ± 0.04b

5.8 ± 0.05a

52.5 ± 0.55B 3.7 ± 0.30 3.4 ± 0.22

55.5 ± 0.39A 3.2 ± 0.32 3.3 ± 0.21

74.98 ± 0.180B 22.64 ± 0.98 1.17 ± 0.084 1.25 ± 0.16 0.33 ± 0.032

75.61 ± 0.088A 22.16 ± 0.107 1.04 ± 0.099 1.21 ± 0.015 0.27 ± 0.031

±: Standard error of the least squares means. A, B : Means in the same row with no common superscripts differ significantly (P b 0.01). a, b : Means in the same row with no common superscripts differ significantly (P b 0.05).

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reserve, thereby limiting meat acidification (D'Agata et al., 2009; Szendrő et al., 2009). Meat colour traits and cooking losses measured on LL muscle were not significantly modified by changes in group size, whereas BF muscle showed greater L* values in the 16 and 4 rabbits/cage groups than in the 8 rabbits/cage group (P b 0.01). This result is unclear and is not supported by other studies (Szendrő & Dalle Zotte, 2011). For this reason, the effect of group size on meat colour traits requires further study. The hind leg meat of the 16 rabbits/cage group was characterized by the highest water content (P b 0.01). The slight reduction in lipids with increased group size could be related to the enhanced locomotory behaviour linked to the greater floor space available in accordance with the findings of Combes et al. (2010); Dal Bosco et al. (2002); Metzger et al. (2003) and Szendrő et al. (2009). 4. Conclusions The results of the present study indicate that the stocking densities lower than 16 rabbits/m 2 did not improve the growth performance, carcass traits and meat quality in the local population reared under outdoor housing system. Since the number of rabbits used in the trial was slightly low, further studies are needed for scientifically based results. Group size had a much greater effect on the variables considered in this study: in general, and also as reported for conventional rabbits, large-group housing has more disadvantages than advantages also for slow-growing rabbits. Experimental evidence showed that larger group size decreased the growth performance as a result of chronic stress attributable to either aggressive behaviour or high locomotory activity related to the larger available area. In conclusion, the best combination of density, group size and total available surface that showed the best production and carcass traits in the considered local population of rabbits reared outdoor was of 5 rabbits/m 2, 4 rabbits/cage and 0.8 m 2. However, these results, and particularly those related to growth performance, need to be fortified by further studies. References AMSA (1995). Research guidelines for cookery, sensory evaluation and instrumental tenderness measurements of fresh meat. Chicago, Illinois, USA: National Live Stock and Meat Board. AOAC (1995). Official Methods of Analysis (15th ed.). Washington, DC, USA: Association of Official Analytical Chemists. Aubret, J. M., & Duperray, J. (1992). Effect of cage density on the performance and health of the growing rabbit. Journal of Applied Rabbit Research, 15, 656–660. Berzaghi, P., Dalle Zotte, A., Jansson, L. M., & Andrighetto, I. (2005). Near-infrared reflectance spectroscopy as a method to predict chemical composition of breast meat and discriminate between different n-3 feeding sources. Poultry Science, 84, 128–136. Blasco, A., & Ouhayoun, J. (1993). Harmonization of criteria and terminology in rabbit meat research: revised proposal. World Rabbit Science, 4, 93–99. Cavani, C., Bianchi, M., Petracci, M., Toschi, T. G., Parpinello, G. P., Kuzminsky, G., et al. (2004). Influence of open-air rearing on fatty acid composition and sensory properties of rabbit meat. World Rabbit Science, 12, 247–258. CIE, Commission Internationale d'Eclairage (1976). Official Recommendations on uniform color spaces-color difference equations, psycho-metric color terms. Supplement 2 to the CIE publication n. 15. Colorimetry, Paris, France.

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